Electronic pulse filtering system



Nov. 29, 1949 Filed May 24, 1945 E. LABlN El' AL 4 shet's-sheet 1 @ZylDOA/@LD D. GP/EG ELECTRONIC PULSE FILTERING ySYSTEM Filed May 24, 1943 4sheets-sheet 2 @@5934 ggg 1 51 a l-/ /52 :r 41a Il 42 /434 A 171 I WmVlbl/JzfzbKgg :155 (48 ,3 i {an} JJM 55% g2 J4@ 1/415 W42@ Lijm kf 'zm ll l l JIJ I r I il Ll l l Y y zas 5 kan' TURN Nov. 29, 1949 E. LABIN ETAL ELECTRONIC PULSE FILTERING SYSTEM Filed May 24, 1943 4 Sheets-Sheet 3IN VEN TORJ' Vey/05% Nov. 29, 1949 E. LABIN ET AL 2,489,297

ELECTRONIC PULSE FILTERING SYSTEM Filed May 24, 1943 4 Sheets-Sheet 4w/m'H aisee/Minerali 103 HMPL l TUDE DISCR/M/NHME ATTRAZY THPESHULDPatented Nov. 29, 1949 UNTED ST'EE TENT @Fifii ELECTBONEC PLSE FILTERINGSYSTEM Appealing May 24, 1943, serial No. 488,180

16 Claims.

This invention relates to communication systems utilizing trains ofpulses for conveyance of intelligence and more particularly to aselective ltering system therefor.

In the case of pulse communication, either radio or telegraphy, Wherethe pulses are time modulated for transmission of intelligence,interference pulses or pulses of other communicating channels presentwill cause interference and jamming for the usual receiver. In the caseof radio communication an enemy may, for example, attempt to jam thecommunication by utilizing a similar pulse transmission. This imposes onthe same carrier unwanted pulses of one or more shapes. Then again,additional pulses dffering in character such as amplitude, Width, theslopes of leading and trailing edges and/or pulse repetition rate may beadded to the train of pulses as separate channels of communication ormerely to confuse the enemy and render it diflicult for him to determinewhich of the pulses of the wave carries the intelligence which he Wantsto jam.

It is one of the objects of our invention to provide a method and meansto eliminate from a train of pulses those pulses which differ in one ormore shape characteristics from a given pulse shape.

Another object ci our invention is to provide a selective pulsefiltering system capable of eliminating those pulses diiiering inamplitude, width, slope (build-up and decay characteristics of leadingand trailing edges) and/or the repetition rate from wanted pulses of agiven shape and given repetition rate.

Assuming, for example, that a train of pulses is made up of pulsesdiffering in shape characteristics such as amplitude, width and slopeand also in repetition rates, a pulse :filtering system in accordancewith our invention is capable of selecting those pulses of a given pulseshape and given repetition rate from the train of pulses and toeliminate all other pulse shapes and pulses of said given pulse shapeoccurring outside of the given repetition rate. In one embodiment of theinvention energy of the train of pulses is subjected separately inparallel circuits to a plurality of iiltering operations and theresultant thereof mixed and clipped, While in another embodiment thetrain of pulses is subjected in succession to a series of filteringoperations. Each of the iiltering operations operate on a differentshape characteristic or repetition rate to assist in the elimination ofthose pulses diiiering from a given pulse shape or in given repetitionrate.

One of the ltering operations, for example,

2 eliminates those pulses differing in amplitude with respect to a givenamplitude characteristic, a second filtering operation eliminates thosepulses differing in slope from a given slope characteristic, a thirdoperation eliminates those pulses differing in width from a given Widthcharacteristic and a fourth operation eliminates odd pulses not includedin a given pulse repetition rate. The unwanted pulses, of course, maydiffer in one, two or three shape characteristics or only in pulserepetition rate from a Wanted pulse and its :repetition rate. Regardlessof these differences our system operates to eiiectively eliminate theunwanted pulses.

For a further understanding of the invention, reference may be had tothe following detailed description to be read in connection with theaccompanying drawings, in which:

Fig. 1 is a block diagram of one embodiment of our invention;

Figs. 2A, 2B, 2C and 2D are graphical illustrations representing theltering operation of the parts of the system illustrated in Fig. 1;

Fig. 3 is a graphical illustration representing the end result of thecombined filtering operation of the parts of Fig. 1;

Fig. l is a block diagram of a second embodiment of our invention;

Fig. 5 is a graphical illustration of the operating steps of the systemof Fig. 4; and

Fig. 6 is a schematic diagram of the amplitude discriminator, slopeselector and Width discriminator of Fig. 4.

Referring to Fig. 1, the filtering system therein shown is divided intofour branches A, B, C and D connected together in parallel. Each ofthese branches performs a diierent filtering operation. The branch Adiscriminates in amplitude between the pulses applied thereto, branch Bdiscriminates as to diierences in slope, that is, diierences in thebuild-up and decay character of the leading and trailing edges of thepulses, branch C discriminates as to diierences in width, and branch Ddiscriminates according to a given pulse repetition rate.

Referring to the branch A of Fig. l and. to Fig. 2A, assume that a trainof pulses il, i2 and i3 such as shown in curve Ai is applied to thebranch. Pulse l2 is a desired pulse of a given amplitude, while pulse ilis of greater amplitude and pulse i3 is of less amplitude. amplitudediscriminating means of this branch may comprise any suitable amplitudediscriminating means, such for example as disclosed in the copendingapplications of D. D. Grieg Serial No.

561,553, filed November 2, 1944, now United States Patent No. 2,434,921,issued January 27, 1948, and Serial No. 487,071 filed May 15, 1943, nowUnited States Patent No. 2,419,548, issued April 29, 1947, or the systemdisclosed in U. S. Patent No. 2,406,882. For purposes of illustration,however, We choose to show the amplitude discriminator disclosed byGrieg in application Serial No. 487,071, the circuit details of whichare shown in Fig. 6.

When the pulses of curve A1 are applied to the branch A through input 9,the pulses are first subjected to a threshold clipping action by theclipper I whereby the smaller pulse I3 is eliminated by the thresholdclipping level Ill. If the larger pulse were desired, the clipper wouldthen be biased to clip at level |4a.

The output of the threshold clipper I0 retains the pulses I and I2 asshown by curve A2. These pulses are applied to an amplitude selector I5,Figs. 1 and 6, adjustable at |5a to threshold clip the pulses at a levelI6 thereby clipping the upper portion of the larger pulse I. Thisclipped portion is amplified and inverted as shown by curve A3 as pulsela. The output of the amplitude selector I5 is applied to a mixer Il,Fig. 6, together with the output of threshold clipper I0 which issupplied to the mixer Il through connection I8. The pulse energy Ilathus cancels pulse leaving the wanted pulse I2 as indicated by curve A4.

The output of the mixer Il, if desired, may be applied to adifferentiator I9, Fig. 1, thereby translating the pulse I2 intopositive and negative pulses |2a and |2b corresponding respectively tothe leading and trailing edges of the pulse. The mixer |'I may be biasedto provide a threshold clipping operation at a level lla to remove noisefluctuations and other disturbances such as might occur due to thecancelling operation of the larger pulses. As is clear in Fig. 2A, theamplitude discrimination takes place regardless of the shape or width ofthe pulses.

Referring to branch B of Fig. l and Fig. 2B, A.

vwhile pulse 2| has a width corresponding to the base of pulse 22, itsleading and trailing edges are steeper than those of pulse 22. Pulse 23has the leading and trailing edges of greater slope and the base thereofis wider than the base of pulse 22.

The pulses 2|, 22 and 23 are applied to a limit clipper 24 to limit theamplitude of the pulses as indicated by the clipping level 25. Theoutput of the clipper 24 is applied to a differentiator 26 wherebypositive and negative pulses are produced corresponding respectively tothe leading and trailing edges of each of the pulses. Thesedifferentiation pulses, as shown by curve B2, are of amplitudescorresponding to the steepness of the 'corresponding edges of the pulses2|, 22 and 23. Thus, the pulses 2 Ia and 2lb are of greater amplitudethan the pulses 22a and 22|), and the latter are of greater amplitudethan the pulses 23a and 23h. In order to discriminate between thesepulses of different amplitude, the output of the differentiator 26 isapplied to an amplitude discriminator 2'| which may be identical withthe amplitude discriminator of branch A (see circuit parts I5 and ofFig. 6). It follows, therefore,

that by proper clipping of the pulses 2|a or 2lb, as the case may be, apulse output is obtained corresponding to the larger pulse 2 I.

Since it be desirable, however, to obtain a pulse corresponding to thewanted pulse 22 and to eliminate those corresponding to pulses 2| or 23,a clipping operation will be performed at level 29 corresponding to theclipping level I4 in Fig. 2A. This results in carrying forward thepulses according to curve B3. By clipping and inverting the peak of thegreater pulse 2Ib as indicated by the pulse 2|c (curve B4) and mixing itwith the pulses of curve B3, the pulse 2Ib will be eliminated leavingpulse 22h (curve B5) which corresponds to the trailing edge of the pulse22. If desired, the clipping operation may be made on pulses 2|a and 22athereby resulting in an output pulse corresponding to the leading edgeof the pulse 22.

Referring to Fig. 2C and branch C of Fig. 1, let lt be assumed that thewidth discriminator of branch C receives a train of pulses illustratedby curve C1. This curve is shown provided with pulses 3 I 32 and 33 inwhich pulse 32 is the wanted pulse of a given width. Pulse 3| is of lesswidth than pulse 32 while pulse 33 is of greater width. Pulse 3| is alsoshown of greater amplitude than pulse 32 while pulse 33 also differs inthe slope of the leading and trailing edges thereof.

The width discriminator may be any suitable width filtering circuit andpreferably is of the character disclosed in the copending application ofE. Labin, Serial No. 467,509 filed December 1, 1942, now United StatesPatent No. 2,418,127, issued April 1, 1947. The pulses of curve C1 areapplied to a differentiator 34 thereby translating the pulses 3|, 32 and33 into positive and negative pulses illustrated by curve C2. The pulsesproduced by the differentiation of input pulses 3| and 32 are of thesame amplitude since the build-up and decay slopes of these input pulsesare the same. The pulses 33a and 33b of input pulse 33, however, are ofless amplitude. It follows that a clipping operation could be used toeliminate the pulses 33a and 33h similarly as in the case of the pulsesillustrated in Fig. 2B. This, however, would leave pulses 3|a, 3|b andpulses 32a, 32h without any other diierenceexcept the timingtherebetween.

According to the method selected for purposes of illustration, widthdiscrimination is made by applying the output of the differentiator 34to an invertor 35 and then through a delay device 36 f which is adjustedto retard the pulses an amount t1 corresponding to the width of thewanted pulse 32. This inversion and retardation effect is illustrated bycurve C3. The pulse output of the delay device 36 (curve C3) togetherwith the pulse output of the differentiator 34 (curve C2) are applied toa mixer and clipper stage 38. Curve C4. illustrates the mixedrelationship of the pulses. It will be noted that in this mixingoperation only those pulses 32a and 32h are in alignment and thereforeare the only pulses of the two curves that add together as indicated at32e. The stage 38 is preferably biased to clip the pulses at a level 39,thereby selecting pulse 32e and eliminating all the other pulses. Ittherefore follows that by this method all pulses differing in width froma given pulse shape are eliminated and a new pulse 32e is producedcorresponding to the trailing edge of the pulse of given width.

If an output pulse corresponding to the leadrngedgeio the wanted pulse,is desired, thisV may llaaccornplished` by inverting the pulses of curveG2. and mixing same with p ulses according to curve C2 delayed a periodequal to the period of the Wanted pulse. This will result in a largepulse corresponding to the leading edge of the wanted pulse and byclipping all other pulses canbeeliminated.

Referring now to Fig. 2D and branch D of Fig. l-let it be assumed thatthe pulses fila, Mb, 42 and; 42a ofcurve D are applied to the input of'lllllch` D. The pulses ela, 4I?) and 42a are the wanted pulses and theyare shown to occur at a given repetition rate. The blocking feature ofthe branch D is ofA special utility for eliminating otherI pulses. ofshape identical to the wanted pulses, Vwhichoccur at a repetition ratediffering from the repetition,- rate of the wanted pulses. The methodfollowed in carrying out this blocking operation may be in accordancewith known method or those disclosed in the copending ap plication of H.G. Busignies Serial No. 380,186, led February 24, 1941, now UnitedStates Patent No. 2,423,082 issued July 1, 1947, and U. S. Batent No.2,406,019. The pulses of curve D1 are first applied to the couplingstage 44. The output of the coupling stage de is applied to a selectorcircuit 45. The selector circuit is tuned tov a period corresponding tothe repetition rate of the wanted pulses so as to produce a suitableharmonic. By suitable adjustment, the selector means 45. and the squarewave generator 46 may be caused to produce from this harmonic a blockingpotential occurring only when no wanted pulses are due for reception.This blocking potential is illustrated by the potential line 41. Whenthe blocking potential drops below the zero axis of curve D1, as`indicated at 48, a, n egative blocking potential is produced at theoutput of the square wave generator which is applied by connection 49 tothe coupling stage 44. This blocking potential will thereby eliminatepulses such asthe pulse 42 occurring between the wanted pulses. Thewanted pulses thus passed by the coupling stage 44 are applied overconnection 50 to a diierentiator 5l by which the pulses are translatedinto positive and negative pulses shown by curve D3.

In Fig. 3, curve rc, we have shown an input train of pulses which, letit be assumed, is applied at theinput 9 to theltering system of Fig. 1.The p ulses of thistrain are selected of various shape characteristicsto illustrate the filtering function ofthe several branches of theliltering system. The train comprises pulses Si, 62, 63, 64, 65 and 6,6:Assume that pulse 62 is the wanted pulse havinga given amplitude, givenbuild-up and decay characteristics and a given width. The p ulse 6Idiffers both in amplitude and width from the wanted pulse 62. Pulse 63diiers in build-uprand decay slopes from pulse 52 and pulse64 dierstherefrom in width. The pulse 65- is identical to the shape of pulse E32but is outsideof a given repetition rate of pulse E52. Pulse 66l differsfrom pulse 52 in that it is of smaller amplitude and of greater width.

Curve sa represents the output of the branch A after the pulses arefiltered according to amplitude (see Fig. 2A). Curve 3b represents theoutput of the branch B after the pulses have been filtered according tothe buildup and decay slope vcharacteristics of the wanted pulse 62(Fig. 2B).

It Awill be seen'that only those pulses having the corresponding slopesremain at the output of thebranch B., Curve 3c represents the output.

UT u',

1 lector of branch C in accordance with the width dscrimination (seeFig. 2C). Curve 3c, therefore, has pulses corresponding only to theinput pulses of a width equal tothe width of pulse 6.2. Curve representsthe output of branch D in accordence with the given repetition rate ofpulse 52 (see Fig. 2D). This blocking effect is indicated by theblocking potential 68 and it will be seen that pulse 65' is therebyeliminated. This blocking feature may be adjusted to eliminate the otherpulses 5l, 63., t4 and 66 in branch D, although not necessarily so.

Curve 3e represents the output o the several branches as they appear inmixed relation in the mixer and threshold clipper l0. It will be seenfrom the alignment of pulsations of curves 3a, 3b, te' and iid that thepulsationsV corresponding to the trailing edge of wanted pulses 52 aregreater in number than for` any of the pulsations corresponding to theunwanted pulses. The threshold clipping function of the mixer l0 maytherefore be adjusted to clip only the largest pulsations which in thiscase are pulses 52e. The final output is represented by curve 3f.

It is tnus clear that according to this embodiment of our invention allpulses diiering in shape characteristics as Well as repetition rate froma given pulse shape and a given repetition rate are eliminated therebyreducing interference to only pulses or disturbances that are insuperposition of a wanted pulse. Since this superpositioning ofinterfering pulses is infrequent such interference be disregarded oratleast easily overcome by repeating signals.

In the embodiment shown in Fig. 1, the several shape discriminatingstages are arranged in parallel and the outputs thereof are combined andclipped to obtain the wanted signal increments. In Figs. 4, 5 and 6, asecond embodiment is illustrated in which the several pulsediscriminating stages are arranged in tandem. The consecutive stages ofthis system comprise an amplitude discriminator lili, a slope selectorE02, a width discriminator |03 and a repetition rate seltlf. Each ofthese stages is adjustable so that a pulse of given shape, that is ofgiven amplitude, given slope characteristics, given width and therepetition rate of which is known may be selected. For a discussion ofthe application of the pulse amplitude, slope and width discriminatingfeatures of this embodiment as an interference limiter in radioreceivers reference may be had to our copending application Serial No,488,182, filed May 24, 1943, now United States Patent No. 2,434,937,issued January 27, 1948.

Let it be assumed, for example, that a train of pulses lll. H2, H3, H4and H5 such as shown, by curve 5a of Fig. 5 is applied to the input lifi of the amplitude discriminator lill. Also yassume that pulse l l2 isthe given pulse shape of the wanted pulses. The amplitude discriminatoris preferably selected of the character disclosed in branch A of Fig. l.According to the clipping features of the discriminator of branch A.-the pulses are rst clipped at a level HS to eliminate the smaller pulsessuch as pulse l i3 and second. at a level to clip the larger pulses forinversion and mixing whereby the larger pulses such as pulse lll areeliminated. Curve 5b, therefore, represents the output of the amplitudediscriminator lill after the larger and smaller pulses Ill and H3 ofcurve 5a are eliminated. It will be noted that -by this amplitudediscrimination, pulse !|5.has been re-shaped increasing 7 the steepnessof the edges thereof las indicated at ||'5b.

The slope selecting stage |02 may be of any known gate clippingcharacter capable of clipping the pulses between two selected levels.For illustration purpose-s we show in Fig. 6 a clipper gate of thischaracter which is also disclosed in our copending application SerialNo. 488,182. Assuming that pulse ||2 has the given slopecharacteristics, the stage |02 will be adjusted at |02a and |2b to slicethe pulses between limits such as indicated by lines |31 and |32 Wherepulse llb, for example, differs in width from pulse ||2.

The width discriminator |03 is of the adjustable L-C damped circuitcharacter disclosed in our copending application Serial No. 488,182. Asdescribed in detail in our aforesaid copending application, the widthdiscriminator first inverts and amplies at lila the pulse energy toprovide negative pulses as shown by curve 5c. It will be noted that byslicing pulse ||5b and amplifying, 'a pulse l |50 substantiallyrectangular in shape s produced. These negative pulses are then appliedto the L-C circuit to produce undulations according to curve 5d. Wherethe L-C circuit is tuned to a period twice the time interval between theleading and trailing edges of pulse I2, the shock excitation caused bythe leading and trailing edges will produce oscillations 12| and |22Which Iare in step and therefore produce a maximum undulation ll2d. Thepulses Ill and l l5, however, are of larger and smaller periodsrespectively than the period of this tuning adjustment. The largerperiod of pulse H13 produces oscillations which are out of step andtherefore form an undulation Hdd which is of amplitude less than theamplitude of undulations ||2d. The period of pulse shape l |519 beingsmaller than the period of pulse H2 also produces an undulation ||5dwhich is of smaller amplitude. By suitably clipping at a level |25 byclipper tube |03b, a pulsation |2e is obtained corresponding to thewanted pulse l I2. The damper tube |030 is maintained inoperative by theinput pulse energy applied to the control grid |0301 After the durationof the input pulse energy the tube ||J3c is permitted to conduct whenoscillations in the L-C circuit swing to negative polarity therebydamping out the oscillatory energy following undulations llZd, Hdd andll'd as indicated in 'curve 5d.

A Should it be desired to obtain a pulsation corresponding to the pulsel5 and to eliminate the other pulses of the curve 5a., the slopeselecting stage EQ2 will be adjusted substantially as above to gate clipthe pulses of curve 5b between limits |3| and |32. By widthdiscrimination of the resulting pulse shapes of curve 5c with the L-Ccircuit for stage H53 tuned at a period which is twice the period ofpulse l lc, a series of undulations l iZf, l Ulf and l 55j differentfrom the corresponding undulations of curve 5d is produced. It will beobserved that therundulation |5f is of greater amplitude than theundulations |21' and Hilf. By a clipping operation the pulsation |I5gmay be segregated from the other undulations.

The additional undulation |34 adjacent undulation Hilf is theoscillationproduced by the leading edge of the pulse.

The repetition rate selector stage Ill@ is shown yin tandem relationwith respect to the shape discriminator stages of Fig. 4.and may be usedto eliminate pulses identical in shape with the 'Wanted pulse Where theyoccur outside of the 8 given repetition rate of the Wanted pulse. Sincethis is illustrated graphically in Figs. 2D and 3, a furtherillustration yand description of the function thereof is believedunnecessary.

It will be observed that the width discriminator feature of Fig. 4 isconsiderably different from the width discriminator C of Fig. 1. Shouldit be desirable, the width selector |03 may be substituted for the widthdiscriminator C. Should the substitution be made, it will be necessary,however, to provide a delay device for the output from branches A and B`so as to retard the pulse output of these branches an amountcorresponding to the retardation of the undulations produced by thewidth discriminator in response to the input pulses. The repetition ratediscriminator could, in that case, be used in series with the miXerstage 'it similarly as in the series arrangement of Fig. 4.

While we have shown and described the principles of our invention inconnection with specific embodiments, we recognize that various changesand modifications may be made therein without departing from theinvention. For example, any two or more of the discriminating stages maybe used together in the manner illustrated where a lesser number ofshape characteristics are involved and Where pulse repetition rate isnot an important factor. It will be understood, there'- fore, that theseembodiments are given by way of example only and not as limiting theobjects of our invention and the appended claims.

We claim:

1. A method of selectively ltering a train o pulses to eliminate thosepulses differing in shape characteristics from a given pulse shapecomprising subjecting energy of the train of pulses independently to aplurality of filtering operations, each of said filtering operationsbeing such as to eliminate those pulses differing in one of saidcharacteristics from said given pulse shape, mixing the remaining pulsesfrom each of the filtering operations whereby the pulse energy passed bythe filtering operations is greater for pulses of the given pulse shapethan for pulses of other shapes, and clipping the greater pulse energythus obtained thereby eliminating the energy corresponding to thosepulses differing from said given pulse shape.

2. The method defined in claim 1 wherein said given pulse shape hasgiven amplitude and slope characteristics, and the filtering operationsare such as to eliminate those pulses differing in at least a selectedone of said characteristics.

3. The method defined in claim 1 wherein said given pulse shape hasgiven amplitude and width characteristics, and the filtering operationsare such as to eliminate those pulses differing in at least a selectedone of said given characteristics.

4. The method dened in claim l wherein the given pulse shape has a givenbase width characteristic and the edges thereof have given slopecharacteristics, and the filtering operations are such as to eliminatethose pulses differing in at 65 least a selected one of thesecharacteristics.

5. The method defined in claim 1 wherein the given pulse shape has givenamplitude, width and slope characteristics, and the filtering operationsare such as to eliminate those pulses differing 70 from said given pulseshape in at least a selected one of said characteristics.

6. A system for selectively filtering a train of pulses to eliminatethose pulses diiiering in shape characteristics from a given pulse shapecom- 75 prising a plurality of pulses filtering means,

means connecting said filtering means in parallel so that each operatesindependently on input energy of the train of pulses, each of saidiiltering means being such as to eliminate those pulses diiering in oneof said characteristics from said given pulse shape, a mixer, means tosupply the output of said iiltering means to said mixer, whereby pulseenergy corresponding to the characteristics of said given pulse shapepassed by the ltering operations of said ltering means combine, thepulses corresponding in all of the given shape characteristics producinga combined energy greater than those pulses differing from one or moreof the given shape characteristics, and means for clipping the greaterpulse energy thus obtained thereby eliminating the energy correspondingto those pulses diiering from said given pulse shape.

7. The system deiined in claim 6' wherein the filtering means comprisean amplitude discriminator stage, a slope discriminator stage and aWidth discriminator stage.

8. The system defined in claim 6 in combination with la pulse repetitionrate discriminator arranged to eliminate pulses identical in shape withsaid given pulse shape but occurring outside of a given repetition rate.

9. The system dened in claim 6 wherein there are at least two lteringmeans, one of said filtering means being arranged to eliminate thosepulses differing from the amplitude characteristic of said given pulseshape and the other of said ltering means being arranged to eliminatethose pulses differing from the width characteristic of said given pulseshape.

10. The system dened in claim 6 wherein there are at least two filteringmeans, one of said filtering means being arranged to eliminate thosepulses diiering from the amplitude characteristic of said given pulseshape and the other of said ltering means being arranged to eliminatethose pulses diiering from the slope characteristic of said given pulseshape.

11. The system defined in claim 6 wherein there are at least twofiltering means, one of said filtering means being arranged to eliminatethose pulses differing from the slope characteristic of said given pulseshape and the other of said filtering means being arranged to eliminatethose pulses differing from the width characteristic of said given pulseshape.

12. The system defined in claim 6 wherein the 10 iiltering meanscomprise an amplitude discriminator means, a slope discriminator means,a width discriminator means and a pulse repetition discriminator means.

13. A system for selectively filtering a train of pulses to eliminatethose pulses differing from one or more of the given amplitude, givenslope and given width characteristics of a given pulse shape, comprisingfilter means to pass energy of those pulses having said given amplitudecharacteristic, filter means to pass energy of those pulses having saidgiven slope characteristic, lter means to pass energy of those pulseshaving said given width characteristic, and means to combine theoperations of the several filter means to produce a nal unitary sequenceof output pulses having a timing according to those pulses of said trainhaving said various given shape characteristics.

14. The system dened in claim 13 wherein the means for combining theoperations of the several filtering means includes means for connectingsaid lter means in tandem relation whereby certain of said lter meansoperate on the pulse energy output of other of said filter means.

15. The system defined in claim 13 wherein the means for combining theoperation of the several lter means includes means for connecting saidlter means in parallel circuit relation for separate operation on thepulses of said train and means for combining the output of the severalfilter means for producing therefrom said nal output pulses.

16. The system dened in claim 13 in combination with a pulse repetitionrate discriminator for eliminating pulses of said given pulse shapeoccurring outside of a given repetition rate.

EMILE LABIN. DONALD D. GRIEG.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,483,172 Gannett Feb. 12, 19242,132,655 Smith Oct. 11, 1938 2,151,149 Pach Mar. 21, 1939 2,211,942White Aug. 20, 1940 2,231,792 Bingley Feb. 11, 1941 2,284,714 BedfordJune 2, 1942

